Activists say it is the first time Islamic State militants have used such a method to perform executions.

The ancient temple of Baal Shamin, or "Lord of the Heavens," in Palmyra, Syria, before it was destroyed by Islamic State militants in July 2015. On Oct. 27, 2015, activists reported IS militants executed three prisoners in Palmyra by tying them to columns and blowing them up. Photo: Syria's department of antiquities and museums

Islamic State supporters posted photos of the destruction of the ancient temple of Baal Shamin, or "Lord of the Heavens," in Palmyra, Syria. Syria's head of antiquities confirmed the photos were authentic. On Oct. 27, 2015, activists reported IS militants executed three prisoners in Palmyra by tying them to columns and blowing them up. Photo: Syria's department of antiquities and museums

PALMYRA, Syria, Oct. 27 (UPI) -- The Islamic State detonated explosives on three prisoners strapped to columns in the ancient ruins of Palmyra, Syria, according to activists.

The Syrian Observatory for Human Rights, a Britain-based group monitoring the war in Syria, reports the identities of the prisoners are not known but notes it is the first time IS militants have used such a method to execute detainees.

IS forces captured the ruins in May along with the adjacent city of Tadmur, and since then the militants have destroyed several temples, tombs, towers and arches at the site, which dates back thousands of years.

IS has also destroyed multiple ancient sites in Iraq, saying the artifacts represent "idolatry," but officials say the group also sells such items on the black market to fund operations.

In August, IS publicly beheaded 82-year-old Khalid al-Asaad, a renowned scholar of antiquities who had worked as the director general of the Palmyra Directorate of Antiquities and Museums for over 50 years. Asaad had reportedly refused to reveal the location of various artifacts sought by IS.

SOHR reports IS has been experimenting with new ways to execute prisoners in recent months, including driving a tank over a Syrian soldier who the militants allege did the same to dead IS fighters. IS has also reportedly killed detainees after forcing them to dig graves with their own hands.

On Tuesday warplanes bombed portions of Palmyra, SOHR reports, noting the Syrian military also conducted airstrikes in the vicinity of the ruins. The activist group did not clarify which country the warplanes came from, but Russia has since Sept. 30 conducted airstrikes on behalf of its regional ally, Syrian President Bashar al-Assad.